Editing GC-content (section)

=== Among-genome variation === 
GC content is found to be variable with different organisms, the process of which is envisaged to be contributed to by variation in [[Gene-centered view of evolution|selection]], mutational bias, and biased recombination-associated [[DNA repair]].<ref>{{cite journal |author=Birdsell JA |title=Integrating genomics, bioinformatics, and classical genetics to study the effects of recombination on genome evolution |journal=Mol. Biol. Evol. |volume=19 |issue=7 |pages=1181–97 |date=1 July 2002|pmid=12082137 |url=http://mbe.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=12082137 |doi=10.1093/oxfordjournals.molbev.a004176|citeseerx=10.1.1.337.1535 }}</ref>

The average GC-content in human genomes ranges from 35% to 60% across 100-Kb fragments, with a mean of 41%.<ref name="IHSGC2001">{{cite journal |author=International Human Genome Sequencing Consortium | title = Initial sequencing and analysis of the human genome | journal = Nature | volume = 409 | issue = 6822 | pages = 860–921 | date = Feb 2001 | pmid = 11237011 | doi = 10.1038/35057062 | bibcode = 2001Natur.409..860L | doi-access = free | hdl = 2027.42/62798 | hdl-access = free }} (page 876)</ref><!--The "Romiguier2010" paper provides a mean GC level at the third codon position in genes. Since protein coding regions are massively overrepresented in GC-rich DNA, their number (46%) is much higher than the genomic mean.--> The GC-content of [[Yeast]] (''[[Saccharomyces cerevisiae]]'') is 38%,<ref>[https://www.ncbi.nlm.nih.gov/sites/entrez?db=genomeprj&cmd=Retrieve&dopt=Overview&list_uids=128 Whole genome data of ''Saccharomyces cerevisiae'' on NCBI]</ref> and that of another common [[model organism]], thale cress (''[[Arabidopsis thaliana]]''), is 36%.<ref>[https://www.ncbi.nlm.nih.gov/sites/entrez?db=genomeprj&cmd=Retrieve&dopt=Overview&list_uids=116 Whole genome data of '' Arabidopsis thaliana'' on NCBI]</ref> Because of the nature of the [[genetic code]], it is virtually impossible for an organism to have a genome with a GC-content approaching either 0% or 100%. However, a species with an extremely low GC-content is ''[[Plasmodium falciparum]]'' (GC% = ~20%),<ref>[https://www.ncbi.nlm.nih.gov/sites/entrez?db=genomeprj&cmd=Retrieve&dopt=Overview&list_uids=148 Whole genome data of ''Plasmodium falciparum'' on NCBI]</ref> and it is usually common to refer to such examples as being AT-rich instead of GC-poor.<ref>{{cite journal |vauthors=Musto H, Cacciò S, Rodríguez-Maseda H, Bernardi G |title=Compositional constraints in the extremely GC-poor genome of ''Plasmodium falciparum'' |journal=Mem. Inst. Oswaldo Cruz |volume=92 |issue=6 |pages=835–41 |year=1997 |pmid=9566216 |url=http://www.scielo.br/pdf/mioc/v92n6/3431.pdf |doi=10.1590/S0074-02761997000600020|doi-access=free }}</ref>

Several mammalian species (e.g., [[shrew]], [[microbat]], [[tenrec]], [[rabbit]]) have independently undergone a marked increase in the GC-content of their genes. These GC-content changes are correlated with species [[Phenotypic trait|life-history traits]] (e.g., body mass or longevity) and [[genome size]],<ref name="Romiguier2010">{{Cite journal|last1=Romiguier|first1=Jonathan|last2=Ranwez|first2=Vincent|last3=Douzery|first3=Emmanuel J. P.|last4=Galtier|first4=Nicolas|date=2010-08-01|title=Contrasting GC-content dynamics across 33 mammalian genomes: Relationship with life-history traits and chromosome sizes|journal=Genome Research|language=en|volume=20|issue=8|pages=1001–1009|doi=10.1101/gr.104372.109|issn=1088-9051|pmc=2909565|pmid=20530252}}</ref>  and might be linked to a molecular phenomenon called the GC-biased [[gene conversion]].<ref name=Duret2009>{{cite journal |vauthors=Duret L, Galtier N |s2cid=9126286 |title=Biased gene conversion and the evolution of mammalian genomic landscapes |journal=Annu Rev Genom Hum Genet |volume=10 |pages=285–311 |year=2009 |pmid=19630562 |doi=10.1146/annurev-genom-082908-150001 }}</ref>